Method for determining hardenability of steel



July 3, 1951 M. A. GROSSMANN METHOD FOR DETERMINING HARDENABILITY OF STEEL Original Filed June 25, 1942 3 Sheets-Sheet l L4 L6 L8 20 2.4 2.6 2.8 2.0 D1 WILL 5.

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PatentedJuly 3, 1951 r UNITED s'TATEs ATENT -OFFICE METHOD FOR DETERMINING. HARDENABILITY OF STEEL Marcus A. Grossmann, Pittsburgh, Pa., assignor to United States Steel Company, a corporation of New Jersey Original application June 25, 1942, Serial No. 448,453. Divided and this application May 31, 1945, Serial No. 596,921

This invention relates to a method of determining in advance the hardenability of an alloy steel of a given composition or, conversely, of determining what alloy elements and how much of each .will be necessary to produce a steel hav- It is well known that identical quenching treatments cause heavy sections of diiferent alloy steels to harden to various depths. Measuring hardness values from the surface to the center of corresponding specimens shows that the loca- 1 Claim. (c 73-15) tion of points of a given hardness may vary withpenetration and other characteristics of 'immediate interest to steel users.

' of a desired alloying element are added to the metal of the same heat while being teemed into for the whole series, of additions.

found that the composition of the base metal has substantially no influence on the resultsobtained,

mined, according to the present invention, by

evaluatin the individual eifect of each component of steel, including so-called residual elements, on hardness penetration, totalling such effects, and adjusting this total eifecton hardenability in the light of grain size of the steel.

In carrying out the invention, the individual effect of elements composing steel on hardenability is evaluated by alloying with molten-steel different amounts of the elements under con-' sideration and determining their influence on In the preferred hardenability characteristics. method of this invention, the required amounts successive ingot molds. This practice is followedin order to assure the identicity of the base used It has been provided it does not vary within any one series.

It has been recognized for a long time thatharden 'lity is a function of several factors, in clud g among them the grain size and alloying effect of these factors on the response of steel to hardening, and to develop suitable generalizations for predicting the hardenability of a given constituents of steel. A large amount of .work. has been done in attempts to determine the exact Alloy ingots cast in the above manner are allowed to solidify and then subjected to conven- 'tional manufacturing steps for bringing them to the final dimensions suitable for specimens to be r Specimens are preferably homogenized by an appropriate used inactu'al hardening processes.

preliminary heat treatment and machined to meet the requirements'of surface and dimensions imposed by the specifications of the hardening test; After being heated to the desired criticaltemperature under conditions reducing oxidation.

- ofthe surface, specimens are quenched in a bath "underconstant conditions.

steel without actually making the steel and sub jecting specimens to hardening. Certain of these generalizations, expressed in formulas relating h'ardenability to grain size and composition, per

mitted evaluation of this property of steel in advance of manufacture and test, with reasonable accuracy, but only within a limited range of factors.

suggest a formula, or propose a law, embracing this relation within the whole field of industrial applications. N0 method has been known for determining this relation, and no method was available for determining hardenability of a steel-'1 in advance of actual quenching of specimens thereof. I

An object of the present invention i a method for determining the relationbetween grain size,v composition, and hardenability of steel.

A related object of the present invention is a method for predetermining hardenability of steel from its composition and grain size.

The desired relation between composition an grain size of steel and its hardenabilit is deterfor-many years for judging the depth of harden-- j Quenched specimens, commonly round bars, are' sectioned and the extent of hardness penetration determined. Since the boundary line between hardened rim and unhardened core has been used ing and sincethe composition of the steel at this boundary line corresponds to 50 per cent martensit'e, .the 50 per cent martensite concentration'is Investigators failed to discover any universally applicable generalization for the relation; between different factors andhardenability or to e 'usedin the present method as the criterion of hardnessjpenetration' and the criterion of hardjenability adopted in the present method is the '-distanceffrom the surface of specimen at which the'steel contains 50 per cent martensite.

1 It is, however, more advantageous, from the standpoint of experimental convenience, to express hardenability in terms of ideal critical diameter, which is the diameter of the bar in which the hardened rim extends just to the center, or in which the unhardened core is just absent after the bars have been. cooled instantaneously .to

room temperature. This criterion has-aspecial advantage in practical hardenability determination,v since the diameter at which the unhardened coreis small is very sensitive to changes in "hardenability. The ideal critical diameter Dr is a true measure ofhardenability alone, since it refers to a constant severity of quench, it can be accurately determined, is applicable both to high and low hardenability, is related to already familiar manner of testing by depth of hardening, and can be readily visualized for practical applications being the size which will just harden throughout in the severest possible quench.

Actual hardness measurements have been found inadequate for determining hardenability within the meaning of the present invention. Hardenability depends by its definition on the location of the 50 per cent martensite boundary line. Experience has demonstrated that even at the 50 per cent martensite boundry there are, wide variations of actual hardness. This hardness ranges in plain carbon steels from substantially32 Rockwell C for a 0.20% C steel to about 55 Rockwell C for a 0.90% C steel, showing even greater variations when alloying elements are present in the metal.

A combination of hardness measurements with certain additional steps, however, ofiers fully adequate means for hardenability determination. Properly quenched specimens are sectioned trans versely to their long axes, and hardness determinations are made at small intervals from the surface to the center. A hardness-testing device similar to Rockwell C hardnesstester has been found satisfactory for the purpose, and onesixteenth-inch intervals provide a suflicient closeness of measurements without unduly complicating testing operations. Hardness figures are recorded and plotted against the distance from the surface. An inflection in the curves so produced indicates the area, when present, of a 50 per cent martensite alloy. Its presence can be is possible and has been widely used in the olevelopment of the present invention. A series of round bars having progressively increasing dipreviously,.the size of bar that just hardens fully,

ameters is prepared from the experimental steel.

These diameters are selected to provide a minimum diameter sufliciently small to assure hardquench, which is a function of the nature of quenching medium and of the method of its application. Two bars of steel entirely alike in every respect and subjected to quenches of a different severity will result-in a difierent depth of hardening or a diiferent critical diameter. No reliable comparison of critical diameters of tested bars is, therefore, possible unless the severity of the quench actually employed is reduced to some standard value.

The conception of ideal critical diameter underlying the present invention expresses hardenability in terms of the hardenability obtained in the ideal (severest possible) quench. The invention furthermore provides charts similar to that shown in Figure 1 for direct conversion of critical diameter obtained by actual quenching to corresponding ideal critical diameter as a function of severity. of actual quench. In these charts, the ordinates are the values of critical diameter designated D, the abscissae are values of the ideal critical diameter Dr, and the several curves show the relation therebetween for various values of the severity of quench H. The theory and derivation of these charts have been elaborately described by the inventor in the magazine Iron Age of April 25, 1940, pages 25 to 29,

Severity of quench Oil Water Brine No circulation of liquid or agitation 0! piece O. 25 to 0.30 0. 9 to l. 0 Mild circulation (or agitation). 0.30 to 0. 35 1.0 to l. 1 Moderate circulation 0.35 to 0.40 1.2 to 1.3 Good circulation. 0. 4 to 0.5 1.4 to 1.5 Strong circulatiom 0. 5 to 0.8 l. 6 to 2 Violent circulation 0.8 to l. l 4

In the graph shown in Figure 1, the numerical values of D and the numerical values of D1 represent units of length, the units of length being the same for each. Thus, such values may represent inches in one case, millimeters in another, 'and centimeters in still another instance. The top line insuch graph, which is a straight line runningfrom' the zero point to the value of 2.0 for both the D and the D1 values, represents the ideal quench, that is, the instantaneous reduction of the temperature of the specimen to 70? F. which, of course, is impossible of attainment. 'The other curves in such graph represent quenches of varying degrees of severity, such quenches being evaluated numerically in accordance with the table above.

An experimental determination of the influence of phosphorus on the hardenability of steel is, given herebelow as a numerical illustration of the principles involved in the part of the present invention relates to the determination of the eifect of individual elements on the hardenability of steel.

A series of steels having selected increments of phosphorus content was made by adding calculated amounts of powdered 24 per cent ferrophosphorus to the bottom two-thirds of four successive ingots of a heat containing 0.59 C, 0.93 Mn, 0.018% P, 0.18% S, 0.22% Si. These ingots were rolled to 3% square billets, and the two middle billets of each test ingot and of an ingot to which no ferro-phosphorus had been added were used fortesting. Chemical analysis showed these ingots to have the following composition:

Quarter-sections 6" long were cut from these billets, heated for two hours at 1500 F. and cooled in still air to eflfect their normalization. These quarter sections from each ingot were turned into rounds ranging from- "'to 1%" diameter, heated at 1500 F. for two and one-half hours, and quenched vertically in waterto 75-90 F. The

quenched bars were sectioned perpendicularly to their long axis, and the hardness gradient from edge to center was determined for each round by taking Rockwell c readings at intervals on two mutually perpendicular diameters.

The hardness values recorded in this manner were plotted against the distance from the sur-'- face. Then hardness at the center of each bar was plotted against its diameter. The resulting curves are shown in Figure 2 of the drawings, -using circles to indicate the estimated position of the inflection points of the several curves. The relative position of the inflection point shifts to the right with the increasing phosphorus content. 0.020%,it corresponds to a critical diameter of 1 inches, and for steel containing 0.077% phos- 'phorus, the figure obtain is 1H inches. The range of sizes employed was not suflicient to include the critical section of the ingot containing 0.097%

phosphorus. The indications furnished by inflection points on these curves were then checked under the microscope and by fracture observa- For the lowest phosphorus content of' tion s. It-was found that the points of inflection substantially coincided with the presence of 50 per cent martensite in corresponding samples, and to hardness penetration just to the center of bars having corresponding'diameters. These observations were further supported'by conducting a similar series of experiments on steels having a greater hardenabilityand containing 0.63%

carbon, 0.94% manganese, 0.027 I sulphur, 0.027% silicon, 0.02% copper and varying centages of phosphorus.

' The. critical'diameters obtained by the above experiments were converted'to ideal critical diameters by means of the diagram .of Figure 1- and plotted, against phosphorus content, as

shown in Figure 3. Two straight lines, one for I each steel, are drawn to represent the increase in hardenability due to phosphorus. The-slopes of theselines indicate the same increase of hardenability being caused by identical phosphorus percentage in thehigh hardenability and low hardenability steel, while the individual disgres sions from the straightline are less-than 2.5

per cent in terms of the ideal critical diameter.

When the line given in Figure 3 is extrapolated at 0.0% phosphorus, it shows a value of ideal critical diameter equal to 1.574 while} the-same steel eter r 1.980. This increase from 1.574 to 1,980

is an increase of 26 per cent in the-value of ideal criticaldiameter for a phosphorus addition of 0.100%. Since the ideal critical diameter in-.-

7 grain size contained:

effect of phosphorus on hardenability can be expressed by the diagram given in Figure 4.

A comprehensive series of experiments, conducted in a manner described in connection with the determination.- of the hardening effect of phosphorus, was conducted using properly selected steels containing progressively increasing percentages of carbon, manganese, silicon, copper, sulphur, chromium, nickel, vanadium, molybdenum, aluminum, and boron. It has been found that the relation between the percentage of ad ditional elements and the corresponding ideal critical diameter'within the ranges investigated, basically followed a straight-line law. Since the whole concentration range has not been investigated, and since secondary reactions enter the effect of certain alloying elements, the diagrams obtained in the course of these experiments expressing the influence of difierent elements or hardenability given in Figures 5 through 8 occasionally deviate from the straight-line function. They show graphically, however, the effect of alloying-elements concentration on the ideal critical diameter. Figure 5 shows the effect of carbon and grain size on the hardenability of steel. Figure 6 illustrates the influence on hardenability of manganese, chromium, silicon, nickel and copper. Figure 7 shows the eifect on hardenability of moylbdenum, aluminum, phosphorus, sulphur and vanadium. Figure 8 demonstrates the efiect of 'boron on hardenability.

It has been suspected that steel composition and the grain size of austenite about to be quenched are two major controlling factors in the hardenability of steel. Since the'eifects of grain size and composition are present simultaneously .in any observed hardenability behavior, it becomes necessary to distinguish between the two effects. According to the present invention, the separation of the efiect of grain size and determination of the numerical values of the latter have been done following substantially the practice described in connection with the determination of the influence of phosphorus, moditied in the light of specific requirements of the subject.

Steels selected for determining the effect of Steel Carbon Manganese Silicon 0. 43 0. 71 0. 19 0. 0. 76 0. l8 0. 7a 0. 62 o. 21 0. 35 l. 68 0. 22

' mination of the effect of phosphorus on hardenability. 'After normalizing, cylindrical samples were heated at 1430 to 2100 F. and held at the temperature for five hours to impart diilferent at 0.100% phosphorus has an ideal critical diamcreases as a straight linefunction of the phos-.-

phorus content, and this increase amounts to sizesto the austenitic grain composing them.

The heated bars were directly transferred from the high-temperature furnace to a salt bath held just above the upper critical temperature. The

bars were-held in the salt bath for a time suflicient tolower their original temperature to that of the-bath, thus keeping them in a wholly austenitic state while retaining; the grain size correspondingto'the original high temperatures.

15 data are shown in Figure 5 in which ideal critical diameter is plotted against the carbon content of steel as a function of grain size.

The manganese and silicon contents in each of steels A, B, and C above are well within the limits which are considered identical in production practice. The data obtained from following the above procedure on steels A, B, and C were employed in making the curves appearing in Figure 5. The data obtained from similar procedures conducted on steel D, which differs markedly from the other three steels in manganese content, were employed only as a check upon the results shown in Figure 5.

A major contribution of the present invention to the art is the discovery that they total hardening effect of all the alloying elements is the product of the effects of the individual elements. In all formulas proposed hitherto for predetermining hardenability characteristics of steel, the effects of the different elements were considered to be additive. A quantity was assignedto each element, and the sum total of them represented the value sought. The method of the present invention is based on the discovery that a steel composed only of iron and carbon, in which no other element is present, has a certain hardenability, and that each additional alloying element is represented by a factor by which the original hardenability is multiplied. Thus the final hardenability of any steel is the product of the original hardenability of a corresponding iron-carbon alloy multiplied by factors for all the alloying elements present in the steel corrected for its grain size.

As a numerical illustrative example of the teachings of the present invention I explain below the practice followed in determining hardenability of a steel having a grain size of 7 and containing 0.50% carbon, 0.90% manganese, 0.10 silicon, 0.020% phosphorus, 0.029% sulphur, 0.28% nickel, 0.30% chromium, 0.05%,molybdenum and 0.05% copper.

The basic hardenability for this steel is derived from the curves of Figure 5, where point A corresponds to 0.50% carbon and No. 7 grain size. This point corresponds to an ideal critical diameter of 0.24, which is the diameter of a bar which will harden at the center of 50 per cent martensite when its surface is instantaneously cooled to 70 F. This base hardenability is multiplied by a factor specified by 0.90% manganese present in steel, namely by a factor of 4, noted as a point on the ordinate of Figure 6 corresponding to the point B of manganese curve. In the present example, a steel having grain size 7 containing 0.59% carbon and 0.90% manganese, and nothing else, would just harden all the way through when subjected to an ideal quench in a size of 0.24 inch multiplied by 4, namely 0.96 inch, or nearly one inch diameter.

The further addition of 0.10% silicon introduces a multiplying factor of 1.10 corresponding to point C on the silicon curve of Figure 6. The 0.020% phosphorus and 0.029% sulphur introduce two more factors 1.05 and 0.98, respectively, as determined by points D and E respectively on the phosphorus and sulphur curves on the diagram of Figure 7. The nickel content of 0.28 introduces a factor of 1.10, i. e., point F on the nickel curve in Figure 6; 0.30% chromium has a large factor of 1.70 determined at point G on the chromium curve of Figure 6; 0.05% molybdenum supplies a factor of 1.16 and 0.05% copper that of 1.02 determined respectively from point H on molybdenum curve and scaled from the line for nickel in Figure 6.

Multiplying these factors for all elements present in steel results in a figure of 2.40 for the ideal diameter of steel having the above analysis and grain size and actual quenching and microscopic observation check the figure.

The numerical relations discussed above omit from consideration the existence of carbides as a separate phase. When carbides remain undissolved after quenching, the diagrams showing ideal critical diameter as a function of element content can indicate only a maximum possible hardenability. Hardening actually obtained may be much less, depending on the percentage of carbides remaining undissolved. This feature is illustrated by the hatched areas above the chromium and vanadium curves of Figures 6 and 7.

The method of the present invention offers a long standing, 1. e., prediction of hardening characteristics of any steel without actual quenching. It offers a true measure of hardenability independent of quenching conditions, is accurate by resting on precise measurements of actual critical size, is readily applicable to the extremes of the hardenability range, is related to the already familiar depth of hardening method of testing, and is conveniently visualized for practical application through the conception of hardening throughout.

It is believed that many alternates can be evolved around the-teachings of this invention without departing from the spirit thereof as recited in the appended claim.

This application is a division of application Serial No. 448,453, filed June 25, 1942.

I claim:

A method of determining the hardenability of steel, comprising the steps of making a series of steels having the same basic composition but varying in the content of one alloying element, making cylindrical quenching specimens of said steels in various sizes, quenching said specimens under identical conditions, and measuring the diameters of the largest specimen of each of said steels which is hardened throughout, i. e., the critical diameter at which the quenched specimen is characterized by the absence of a relatively soft core within an external hardened case, thereby obtaining, a relation between such critical diameter and the content of said alloying element giving the hardenability factor of the latter, then repeating the procedure set forth above for various elements entering into the composition of steel, the hardenability of a steel containing known amounts of the several elements being given by the product of the hardenability factors corresponding to such amounts of all the elements present in the steel.

MARCUS A. GROSSMANN.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 822,527 France Dec. 31, 1937 OTHER REFERENCES '1938, pages 1-37. (Copy in Div. 3.)

Certificate of Correction Patent No. 2,559,016 July 3, 1951 MARCUS A. GROSSMANN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 4, line 64, for relates read related column 5, line 35, for obtain read obtained; column 7, lines 39 and 40, for 1.10 silicon read 1.10% silicon; line 47, for of 50 per cent read to 50 per cent;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Ofiice. Signed and sealed this 25th day of September, A. D. 1951.

THOMAS F. MURPHY,

Assistant Uommz'ssz'aner of Patenta. 

